1. Field of the Invention
This invention relates to a nonwoven fabric suited for use as an insulating layer, such as a thermal and acoustic insulating layer in an aircraft fuselage.
2. Description of the Related Art
Blankets providing thermal and/or acoustic insulation are used in aircraft and other vehicles to protect passengers from exterior environmental temperature extremes and to dampen engine noise.
As described in U.S. Pat. No. 5,169,700 (“Faced Fiber Glass Insulation”) and U.S. Pat. No. 5,108,821 (“Self-Extinguishing Blanket Enclosed With Plastic Films”), both incorporated herein by reference, prior art insulation blankets typically comprise a batting of a fibrous material such as fiberglass, and a film covering which serves to resist the uptake of moisture by the batting material.
Among the drawbacks of the prior art insulation blankets is that, depending on the materials of construction, the blankets may contribute to the propagation of a fire by providing a pathway into the cabin of the aircraft. Metallized polyethylene terephthalate (PET) films found on such fabric, for example, have been known to propagate fire.
U.S. Pat. No. 5,624,726 and U.S. Pat. No. 5,759,659, herein incorporated by reference, describe an insulation blanket comprising a lofty batting of thermoplastic fibers and a high temperature resistant layer of ceramic oxide fibers, encased within a heat-sealable, flame retardant, rubber-toughened thermoplastic polyolefin polymer. While some of these fabrics may be effective in preventing burn-through, their effectiveness in preventing the propagation of fire when exposed to a flame under a radiant heat source is dependant on the durability of the thin ceramic layer.
U.S. Pat. No. 5,904,318, herein incorporated by reference, discloses a reinforced skin structure containing insulation comprising heat-stabilized, oxidized polyacrylonitrile (PAN) fibers. While PAN fibers work well in preventing burn-through of fire into an aircraft cabin interior, oxidized PAN fiber may alter the pH of condensate contacting it, resulting in accelerated corrosion of the skin of the aircraft. Moreover, toxic gases including cyanogen, carbon monoxide, and nitrogen oxide are possible byproducts of a fire involving nitrogen-containing synthetics such as oxidized PAN.
Insulation blankets made of commodity inorganic fibers such as fiberglass are irritating to the touch and difficult to process on textile equipment. The fibers fracture easily in the process of manufacturing the blanket assembly, during installation, or whenever they are handled.
Commercially available insulation materials which contain fiberglass, glass wool, and other inorganic fibers also typically cause irritation of the skin, eyes, nose, and mouth.
In addition to causing irritation of the skin, there is a health risk associated with glass fibers when broken fragments become airborne. Glass fibers have a propensity to fracture and create dust due to their brittle nature. These fiber fines, when inhaled, pose a serious risk to human health because they are capable of entering the lungs, leading to a chronic condition known as silicosis. For example, a rotary glass fiber commonly used in aircraft insulation has a mean fiber diameter of 4.7 microns and a standard deviation of 2.0 microns. As fibers having a diameter below 3 microns are within the respirable range, a significant portion of these fiber fines is within the respirable range.
Aircraft using conventional insulation blankets accumulate, and frequently fly with, more than a ton of moisture vapor condensate within saturated, soggy, matted and collapsed blankets.
Thus, there has been a long felt need in the industry for an improved insulation material; one which offers improved resistance to fire propagation while overcoming the abovementioned limitations of the prior art.